Over four days, 175 IS members have been killed, Siamand Ali, an SDF spokesperson, told The Telegraph, adding that 27 of his force’s soldiers had been killed.

At least seven civilians have been killed, according to the London-based war monitor the Syrian Observatory for Human Rights.

While hundreds of alleged jihadists have been recaptured, dozens are still on the loose, the observatory claimed.

Across the northeast more than 10,000 alleged members of the terror group, spanning some 50 nationalities, remain in pre-trial detention in facilities run by the Kurdish militia.

Despite British assistance in expanding the jail, authorities have long warned that they do not have enough resources to properly secure so many prisoners in makeshift facilities, and have called on countries to repatriate their citizens.

The calls have largely gone unanswered, leaving analysts little surprised that a fatal jail break finally happened, particularly as IS sleeper cell attacks have increased over recent months.

Children also held at the prison

More than 600 children were also believed to be held in detention at the prison, according to the Rojava Information Centre.

According to the SDF, the insurgents’ operation had been planned over six months. An oil tanker is thought to have breached the jail in a double suicide bombing.

As the clashes spread throughout the town, more than 4,000 civilians were forced to flee their homes over the weekend as the militants began to hide out in residential neighbourhoods.  

IS fighters “are entering homes and killing people,” one fleeing civilian told the AFP news agency.

In a video released by IS at the weekend, the group claimed that it had released hundreds of its members and taken over a weapons storage facility within the prison.

A second video showed 23 captives wearing military fatigues that the group claimed were hostages. The SDF said that they were the kitchen staff from the jail.

Coalition air strikes

The Pentagon confirmed that the US-led coalition had conducted air strikes.

Jailbreaks in Iraq and Syria were a common tactic for the group between 2012 and 2014 as they geared up to claim territory – which at the height of their so-called “caliphate” was the size of Britain.

The group has been severely weakened since its territorial defeat, but the scale of the latest assault and the time it is taking US-backed forces to contain it highlights the lingering danger.

Outside of the prisons, an estimated 60,000 other former residents of the caliphate – mostly women and children – are languishing in camps.